Single-Polarizer Reflective LCDs
Conventional LCDs used in liquid crystal television sets and desktop computer monitors are transmissive displays with polarizers on both sides of the LCD cell. A backlight provides the source of illumination. Such two-polarizer transmissive LCDs can be transformed into two-polarizer reflective LCDs simply by placing a reflector behind the rear polarizer. Ambient light from the front provides the illumination source. However, there are several problems associated with these two-polarizer reflective LCDs. One problem is the appearance of double-imaging when the display is observed obliquely. This is caused by parallax introduced by the thickness of the rear glass substrate causing the reflected image of the pixels to be displaced from the image formed by the pixels themselves. Double imaging becomes a serious problem for higher resolution reflective displays where the pixel dimensions are comparable to the thickness of the rear substrate plate. Parallax can be avoided if the reflector is placed inside the display cell, but then the rear polarizer would also have to be placed inside the cell on top of the reflector and this is not a practical process. But in a single-polarizer LCD no second, internal polarizer is present and the reflector can be easily placed inside the cell by making rear electrodes out of a reflective metal such as aluminum. Double imaging can thus be completely avoided in single-polarizer reflective LCDs.
Improved brightness is another advantage of single-polarizer reflective LCDs. Practical polarizing films absorb a certain amount of light even when the input light is polarized parallel to the polarizer's transmission axis. Since in a single polarizer reflective display the light passes through a polarizer two times instead of passing through a polarizer four times in a two-polarizer reflective display, the single-polarizer display will necessarily appear brighter. This improved brightness can be an enormous advantage when the display is viewed under conditions of low ambient light.
Finally, a conventional two-polarizer reflective design cannot be used if the rear substrate plate is opaque, such as a silicon chip. In this case the only option is to place the reflector inside the cell. Single-polarizer displays have therefore found broad applications in high resolution Liquid Crystal on Silicon (LCoS) displays used in some rear projection television sets and near-eye headset applications.
Viewing Angle Problem of LCDs
Nematic liquid crystal (LC) molecules form a uniaxial birefringent medium with the optic axis parallel to the director of the liquid crystal. Nearly all known nematic liquid crystals have positive optical anisotropy where the extraordinary refractive index is larger than the ordinary refractive index. The optical properties of LCDs are generally optimized for light passing perpendicularly to the cell substrates. For a light incident at an oblique angle θ, the birefringence of the liquid crystal will modify the state of light polarization in a different way than for light at normal incidence. This could cause, for example, an increased amount of light leakage in the dark state, degrading the contrast ratio at the oblique viewing angle and making the images look washed out. The color of the bright state of the display can also change depending of the oblique viewing angle, giving familiar images an unnatural appearance. The variation of the optical performance of LCDs with viewing angle is an intrinsic problem of LCDs because of the birefringent nature of liquid crystals.
Birefringent Layers
Much work has been done to improve the optical performance of LCDs at oblique viewing angles. This work has primarily focused on using external birefringent layers, known as compensation films, to cancel the retardation of the LCD in the dark state when viewed at oblique angles. A birefringent medium is a medium having anisotropy in refractive index. In the most general case the medium is biaxial and is fully described by the index ellipsoid (FIG. 1). Three different refractive indices (nx,ny,nz) along the three axes (x,y,z) of the ellipsoid define the optical properties of the medium. By convention we choose z axis perpendicular to the glass substrate plane, and x and y axes inside the substrate plane (in planes axes).
When the biaxial media are fabricated, for example, by stretching plastic films, the z axis is perpendicular to the sheet and the x and y axes are in the plane of the sheet. If the two in-plane indices are equal, nx=ny, then the medium is uniaxial and the z axis is the optic axis which is perpendicular to the film plane. In this case the uniaxial film is called a C plate. If either one of the in-plane indices (nx or ny) and the out-of-plane index nz are equal then the film is also uniaxial and the optic axis is parallel to the film plane. In this case the uniaxial film is called an A plate. Such C plate and A plates are illustrated in FIGS. 2a and 2b wherein uniaxial film plates are referenced 1.
A common way to characterize a biaxial medium is through the Nz parameter defined as Nz=(nx−nz)/(nx−ny) with nx>ny. Classically, the axis corresponding to the larger in-plane index (for us nx) is called slow axis and the axis corresponding to smaller in-plane index (for us ny) is called fast axis. The choice nx>ny is an arbitrary choice just to distinguish between the larger and the smaller in-plane index.
Table 1 summarizes the different kinds of birefringent media and their associated Nz values.
TABLE 1relationship between Nz and type of birefringent mediumNz−∞01+∞nz > nx = nynz > nx > nynx = nz > nynx > nz > nynx > ny = nznx > ny > nznx = ny > nzC+BiaxialA−BiaxialA+BiaxialC−
The optical retardation R for a uniaxial medium where, for example nx=ny=no, is defined as R=(nz−no)·d, where d is the thickness of the uniaxial film and no is the ordinary index of refraction. Optical retardation is generally given in units of nanometers, or nm. A biaxial film is characterized by two retardation values, for example (nx−ny)·d and (nx−nz)·d.
Uniaxial A plates and C plates can also be composed of uniformly aligned liquid crystal molecules. Most nematic liquid crystals are uniaxially positive and thus can form A+ or C+ plates depending upon whether the optic axis is oriented parallel or perpendicular to the substrate plane. Discotic liquid crystals are uniaxially negative and thus can form A− or C− plates depending upon whether the optic axis is oriented parallel or perpendicular to the substrate plane. These birefringent oriented liquid crystal layers could either be contained between two substrate plates or they could consist of polymerized liquid crystals forming a free-standing, oriented film of their own. Still other retardation films are known which can be generated by exposing certain types of photopolymers to polarized light which could either be located on the inner surfaces of the substrate plates containing the active liquid crystal material or on the outer surfaces of the substrate plates. Still other retardation films are known which can be deposited on the substrate surfaces through a coating process (Lazarev et al. 1991 SID Digest of Technical Papers p 571-3).
Polarizing films currently available in the market are made of an oriented anisotropically absorbing sheet laminated between two plastic carrier substrates.
Nearly all commercially available polarizers use Tri Acetyl Cellulose (TAC) as a carrier substrate which is known to be a film which is uniaxial or very slightly biaxial having a negative optical anisotropy (Han, Journal of the SID, 3/1,1995 p 15). FIG. 3 shows two TAC substrates 10 and 12 provided on either side of an absorbing layer 14. The optic axis of a TAC film is perpendicular to the plane of the layer, so it is a negative C plate, with typical retardation (nz−no)·d of −40 nm to −55 nm. This type of polarizer is coated with a pressure sensitive adhesive (PSA) and is intended to be laminated on the outside of the substrate plates containing the active liquid crystal material.
Other polarizing films are known which can be coated on inside of the substrate plates containing the liquid crystal material (Ohyama et al. 2004 SID Digest of Technical Papers p 1106-1109). In this case no TAC layer is present. If the polarizer is coated on the inside of the substrate plate it can be located either underneath the electrode or on top of the electrode.
Monostable TN Displays
The most commonly used liquid crystal display mode is the 90° Twisted Nematic (TN) mode. In the absence of an applied electric field the liquid crystal molecules are parallel to the substrate plane and adopt a 90° twisted structure which rotates the plane of polarized light by 90°. When an electric field is applied, the molecules tilt out of the substrate plane according to the field strength, and the layer untwists to a state which does not rotate the plane of polarized light. When such a layer is placed between two crossed polarizers, for example, the display transmission will vary according to the applied voltage. When the field is removed the liquid crystal layer returns to its original 90° twisted texture.
The TN display is an example of a monostable display, which means that if the drive signals are turned off, no image will be displayed and the screen will appear blank because the liquid crystal returns to its single, monostable texture. Monostable displays must be continuously updated in order to see the image, even if the image itself does not change. This is a disadvantage for displays designed for portable equipment since continuously updating the display requires a significant amount of power which puts undue strain on the battery lifetime.
The supertwisted nematic (STN) display is another type of a monostable TN liquid crystal display. It has a twist angle of approximately 240° in the absence of an electric field. Like the 90° TN display, the strength of an applied electric field controls the angle that the liquid crystal molecules tilt out of the substrate plane.
Compensating Monostable TN Displays to Improve Range of Oblique Viewing Angles
In the publication Han, Journal of the SID, Mar.1,1995 p 15 it was noted that the negative optical anisotropy present in the TAC layers of commercial polarizing films actually improved the viewing angle characteristics of the TN display. To further improve the viewing angle characteristics of the TN display Mori proposed to compensate the black, driven state of the display with compensation layers made of polymerized negative birefringent discotic liquid crystal molecules having a nonuniform, splayed optic axis orientation throughout the layer that mirrors the liquid crystal director orientation near the boundaries of the driven TN cell (Mori IDW '96 p 189 or Jpn. J. Appl. Phys. 36 1997 p 143).
STN displays are generally compensated with uniaxially stretched polymer films. The primary purpose of these birefringent films is to remove the intrinsic STN coloration and make it possible for the pixels to switch from black to white—a prerequisite if a full color display is to be realized using color mosaic filters. The compensation films also improve the range of viewing angles, but the range is never as wide as with the compensated TN displays.
Other Monostable Displays and Optical Compensation
Other LCD modes have recently being developed like OCB (Optically Compensated Birefringence) or VA (Vertically Aligned). For example OCB display can be compensated with biaxial film with Nz>1 and specific retardations adapted to the OCB LC mode (C. L Kuo at al, SID 94 p 927-930, see p 928 table 3). To calculate Nz in this publication, it is important to consider the greater index in the substrate plane in the Nz formula:Nz=[n(greater in-plane index)−nz]/[n(greater in-plane index)−n(smaller in-plane index)
In this invention the convention is nx is the greater in plane index and ny is the smaller in plane index, nx>ny (arbitrary choice). In the cited publication, the greater, in plane index is called ny. Thus in the publication, to calculate Nz by applying our formula, it is necessary to change nx by ny and ny by nx. It can be seen that in the publication, the nz value is always smaller than nx and ny, meaning the Nz>1 case. Same case for the publication by Y. Yamaguchi et al (SID 93 p 277-280, table 1 and table 2). In this publication, nx is chosen greater than ny.
It is well known that each LC mode can be optically compensated by dedicated compensation film. Specific parameters of birefringent films have to be optimised for compensation of each LC mode: number and type of birefringent film (uniaxial C or A plate positive or negative, biaxial), retardation values in nm (one retardation value for uniaxial, two retardation values for biaxial), angular orientation of optical axis for uniaxial plates or index ellipsoid axis for biaxial plates. A lot of literature has been published concerning the optical compensation of LC modes, and the CIB class 1/13363 is exclusively dedicated to this topic.
Bistable TN Displays
Another class of nematic LCDs are those that exhibit bistable, multistable or metastable behavior. In this class, the liquid crystal layer can assume at least two distinct textures that are either stable or metastable in the absence of an applied electric field. To switch between the two textures, suitable electronic signals are applied to electrodes that are positioned on each side of the liquid crystal layer. Once a particular texture is created, it persists in the absence of an applied electric field thanks to its bistability. The ability of bistable LCDs to store images in the absence of an applied electric field means that the display does not need to be continuously updated at a high rate if the displayed information does not change. This dramatically reduces the power consumption of the display and makes bistable displays attractive for portable devices where battery lifetime is of paramount importance.
In bistable TN displays the two textures consist of uniformly twisted, substantially planar nematic structures, each having a different twist angle. The basic cell structure of a single-polarizer, reflective bistable twisted nematic (BTN) LCD is shown in FIG. 4a. The cell comprises a layer 38 of chiral nematic liquid crystal, having a uniform twist, between two substrate plates—a rear substrate 30 and a front substrate 22 which faces the viewer 20. Front and rear electrodes 26, 34 positioned on the substrates 22, 30 allow electrical command signals to be applied to the chiral nematic liquid crystal 38 situated between them. A reflector 32 is positioned between the liquid crystal layer 38 and the rear substrate 30. The reflector 32 can be positioned between the transparent electrode 34 and the rear substrate 30, but electrically isolated from the electrode 34. The reflector 32 could, for example, be a multilayer dielectric mirror. The reflector 32 could also be made out of a conducting, reflective material such as aluminum. The electrode 34 and the reflector 32 can be merged in this case. Either of the above designs would not have double-imaging since the reflector would be inside the cell. Alternatively, if the rear electrode 34 and rear substrate 30 are both made of transparent materials, then the reflector 32 could be placed external to the LC cell (FIG. 4b). Alignment layers 28, 36 deposited on the electrodes 26, 34 orient the liquid crystal molecules 38 at the two boundary surfaces to give the desired twist angles for the two bistable textures. A single polarizer 24 situated on the front substrate 22 makes it possible to obtain a bright optical state and a dark optical state, each state being associated with one or the other of the two bistable textures. The polarizer 24 is generally situated external to the LC cell, as shown in FIGS. 4a and 4b, but it could also be situated inside the LC cell. If the display were designed for color, then color filters (not shown) would also be present on the inside of one of the substrates 22, 30. On FIGS. 4a and 4b, the layer thickness of the chiral nematic liquid crystal or cell gap is referenced “d”.
The optical parameters of a single-polarizer, reflective BTN liquid crystal display are (FIG. 5):                the liquid crystal layer retardation Δn·d defined as the product of the liquid crystal birefringence Δn and the cell gap d,        the angle φF that the liquid crystal director 42 on the inner surface of the front substrate makes with the x′-axis. By convention we set the director parallel to the x′-axis, which makes φF=0.        the angle φR that the liquid crystal director 40 on the inner surface of the rear substrate makes with the x′-axis. Angles are defined as positive when the director is rotated away from the x′ axis in a counterclockwise sense.        the polarizer angle P that the absorption axis of the front polarizer 24 makes with the x′ axis. Angles are defined as positive when the absorption axis is rotated away from the x′ axis in a counterclockwise sense.        
Two types of Bistable Twisted Nematic (BTN) displays have been developed that each have two uniformly twisted textures in the absence of an applied field. These two types are known as πBTN displays and 2πBTN displays. They will be described in further detail below.
πBTN Display Called BiNem Display
The πBTN display, also called the BiNem display, has been proposed by Dozov (U.S. Pat. No. 6,327,017). The two stable states of the BiNem display are uniformly twisted nematic textures which, for the case of strong azimuthal anchoring, differ in total twist angle by 180°, hence the name πBTN. Because finite anchoring energy, elasticity or slippage, the two stable states can differ in total twist angle by an angle slightly smaller than 180°, typically between 150° and 180°. Because the two textures are topologically inequivalent, a breaking of the anchoring on one of the surface alignment layers is necessary to switch from one texture to the other (see FIGS. 6a and 6b). This breaking of the anchoring in the BiNem display is obtained by applying an electric field perpendicular to the substrates. Kwok (U.S. Pat. No. 6,784,955) describes a πBTN embodiment where the two textures are switched by an electric field having two components, one parallel and one perpendicular to the substrates.
On FIG. 6a anchoring breaking of the LC molecules is illustrated on the side of a weak anchoring orientation layer 36, the other orientation layer 28 having strong anchoring properties. On the same FIG. 6a, the front transparent Indium-Tin Oxide (ITO) electrode is referenced as 26 and the rear electrode 34 is a reflective metallic material such as aluminum which also serves as the reflector. Another option is to have a transparent electrode 34 and a reflector 32 isolated electrically but both situated between the liquid crystal layer 38 and the rear substrate 30, as shown on FIG. 4a. Alternatively, if the rear electrode 34 and rear substrate 30 are both made of transparent materials, as shown on FIG. 4b, then the reflector 32 could be placed external to the LC cell (FIG. 6b).
The low twisted texture (twist φU) is called the U texture and the high twisted texture (twist φT≈φU±π) is called the T texture. In order to equalize the energy of the two textures, the d/p ratio is given by d/p ≈0.25+φU/2π, where d is the liquid crystal layer thickness or cell gap and p is the intrinsic pitch of the chiral nematic liquid crystal.
Many optimized optical modes for single-polarizer, reflective πBTN displays having both high brightness and high contrast ratio for normal incidence viewing have been proposed by Kwok (US 2003/0076455 A1) and Osterman et al. (Eurodisplay 2002, p. 479-482). These modes have certain layer twist angles, polarizer angles and liquid crystal layer retardations which have been optimized for perpendicular viewing.
Intrinsic Viewing Angle Performance
One of Osterman's single-polarizer, reflective πBTN modes (called mode no. 1-1) is depicted by the optical stack shown in FIG. 7. The optical stack comprises a reflector 32, a liquid crystal layer 38 and a polarizer 24. The viewer 20 is positioned on the opposite side of the reflector with respect to the liquid crystal layer. The polarizer 24 can be either laminated onto the front LCD glass substrate or coated on the inside of the LC cell. The layer twist angles for the U and T textures are −5.7° and 174.3°. The 1-1 mode has a retardation Δn·d of 137.8 nm. There are two possible polarizer angles for mode 1-1: the one presented by Osterman and another one obtained by rotating the polarizer through an angle of 90°. These two polarizer configurations give the same optical performance when the display is viewed perpendicularly, but can give quite different optical performance when the display is viewed obliquely. Since the present invention deals with improving optical performance at oblique viewing, it is imperative to consider both polarizer orientations. Using the sign convention of FIG. 5 the two polarizer angles for Osterman's 1-1 mode are P=−41.3° and P=+48.7°.
The contrast ratio and color shift characteristics for oblique viewing of the display are conveniently represented on conoscopic figures which indicate the contrast ratio and color shift in the form of isolines over the viewing hemisphere. The enclosed figures are simulated using a state-of-the-art simulation software called LCD Master available from Shintech, Japan. For simplicity, the birefringence of the liquid crystal and the compensators is assumed to have no wavelength dispersion and an ideal polarizer and ideal reflector are assumed as well. In the conoscopic figures presented in the present patent the polar viewing angle of incidence is indicated by moving radially in the figure starting at the center for viewing the display straight-on and extending to the periphery for viewing the display at the near grazing polar angle of incidence of 80°. The azimuthal viewing direction is indicated by moving tangentially along the circular lines from 0° to 360°. These diagrams are routinely generated by state-of-the-art LCD simulation software and LCD optical characterization instruments. For contrast ratio, isolines of equal luminous contrast are plotted assuming an equal energy light source, which is an ideal white source having a constant intensity over the visible spectrum. The luminous contrast is defined as the luminous reflectance of the bright state divided by the luminous reflectance of the dark state. The luminous reflectance is the value of the reflectance which has been integrated over the spectral sensitivity curve of the human eye, otherwise known as the photopic reflectance. For color shift, isolines of equal chromaticity difference ΔC are plotted where the reference color is the normal incidence color. One unit of ΔC can be considered to be a just noticeable color difference ΔC is defined by:ΔC=√{square root over ((u*2−u*1)2−(v*2−v*1)2)}{square root over ((u*2−u*1)2−(v*2−v*1)2)},
where the subscript 1 refers to the u* and v* color coordinates at normal incidence and the subscript 2 refers to those coordinates at other oblique angles of incidence. u* and v* are defined by:
            u      *        =          13      ⁢                          ⁢                        L          *                ⁡                  (                                    u              ′                        -                          u              n              ′                                )                                v      *        =          13      ⁢                          ⁢                        L          *                ⁡                  (                                    v              ′                        -                          v              n              ′                                )                                L      *        =                            116          ⁡                      [                          Y                              Y                n                                      ]                                    1          3                    -      16      
where the subscript n refers to the value corresponding to nominally white light. As usual
            u      ′        =                  4        ⁢                                  ⁢        X                    X        +                  15          ⁢                                          ⁢          Y                +                  3          ⁢                                          ⁢          Z                      and            v      ′        =                  9        ⁢                                  ⁢        Y                    X        +                  15          ⁢                                          ⁢          Y                +                  3          ⁢                                          ⁢          Z                    
where X, Y and Z are the tristimulus values.
FIG. 8 shows conoscopic contrast ratio (8a) and conoscopic color shift (8b) diagrams for the example of the single polarizer, reflective configuration illustrated in FIG. 7 where P=−41.3°. On FIG. 8a iso-contrast ratio contours from center outward are: 70, 60, 50, 40, 30, 20 and 10. It is seen from FIG. 8a that a contrast ratio greater than 10:1 over the full 0°-360° azimuthal range is achieved for polar angles θ extending out to a maximum value of 49°. We call this angle θmax. For polar angles larger than θmax there are some azimuthal viewing angles where the contrast ratio is less that 10:1. On FIG. 8b the iso-color difference contours ΔC from center outward are: 2, 4, 6 and 8. In this diagram the maximum color shift, which we call ΔCmax, is 6.7. Of course the complete conoscopic figures give many more details, but much of the important information can be summarized by the two numbers θmax and ΔCmax. This notation will be used throughout the rest of the description of this invention.
This restricted range of viewing angles to achieve a 10:1 contrast ratio for the configuration illustrated in FIG. 7 is adequate, but leaves room for improvement. It would also be desirable to reduce the amount of color shift since a ΔCmax of 6.7 would be quite noticeable.
As mentioned previously, rotating the polarizer by 90° results in the same display performance for perpendicular viewing but different performance when viewed obliquely. Table 2 compares θmax and ΔCmax for these two polarizer configurations where, for this case, the performance is nearly identical.
TABLE 2comparison of the oblique optical performance of mode 1-1 ofthe single-polarizer reflective πBTN display for the twodifferent polarizer configurations.Config. No.P angleθmaxΔCmax#1−41.349°6.7#2+48.748°7.0Influence of TAC Substrates
As explained earlier, commercially available polarizers have birefringent TAC carrier substrates on both sides (FIG. 3) However, only the TAC substrate 12 between the LC layer and the polarizer film 14 will influence the optical performance and needs to be considered.
FIG. 9 shows an optical stack, comprising a polarizer 24 with P=−41.3°, a TAC substrate 25 with a typical TAC retardation value of (nz−no)·d=−55 nm, a πBTN liquid crystal 38 with a retardation of Δn·d=137.8 nm, a U twist=−5.7° and a T twist=174.3° and a reflector 32. Table 3 presents θmax and ΔCmax for the stack of FIG. 9 as well as for the other polarizer angle of 48.7° for comparison. Again, the performance is nearly identical. However in comparing table 3 with table 2 for the intrinsic πBTN display it is seen that the TAC layer significantly worsens the oblique viewing performance by dramatically narrowing θmax and increasing the maximum color shift ΔCmax.
TABLE 3comparison of the oblique optical performance of mode 1-1 ofthe single-polarizer reflective πBTN display with one TAClayer for the two different polarizer configurations.Config. No.P angleθmaxΔCmax#1−41.338°18.6#2+48.738°19.1
FIG. 10 shows the simulated conoscopic figures corresponding to the stack example of FIG. 9. Comparing with FIG. 8 for the intrinsic πBTN is it seen in more detail how the TAC layer significantly narrows the display viewing cone and increases the color shift. This is quite the opposite behavior compared with conventional TN displays (Han, Journal of the SID, 3/1,1995 p 15) where the TAC films actually improve the viewing angle characteristics.
2πBTN Display Called Berreman Display
The 2πBTN display was first introduced by Berreman (U.S. Pat. No. 4,239,345) and later by Tanaka (U.S. Pat. No. 5,900,852). The twist angle of the initial texture is φ+π, and after a reset pulse, two planar metastable textures can be obtained having respectively φ and φ+2π twist for the case of strong azimuthal anchoring of the director at the substrate surfaces. The total twist angles of the two metastable textures thus differ by 2π (see FIG. 11). Switching between the two metastable textures is accomplished by applying an electric field perpendicular to the plane of the substrates. From various studies, it is known that the d/p ratio to achieve bistability in a 2πBTN display is given by d/p≈0.5+φ/2π.
Intrinsic Viewing Angle Performance
Optimized optical modes for single polarizer, reflective 2πBTN displays having both high brightness and high contrast ratio for normal incidence viewing have been proposed by Tang et al. J. Appl. Phys. 87, 632-637 (2000) and Guo et al., Applied Optics 42(19) 3853-3863 (2003). One of these modes, which Guo refers to as mode 1, is illustrated on FIG. 12.
FIG. 12 shows a stack including a polarizer 24 with P=24.24° and a 2πBTN liquid crystal layer 38 with retardation Δn·d=310.8 nm and twist angles of −67.2° and +292.8° and a reflector 32.
Table 4 presents θmax and ΔCmax for the stack of FIG. 12 as well as for the other polarizer angle of −65.76° where the polarizer has been rotated by 90°. Configuration 1 with the 24.24° polarizer angle gives the best performance of the two, but poorer performance in comparison with the intrinsic πBTN performance given table 2. θmax is narrower for the 2πBTN layer (43° vs. 49°) and ΔCmax is much larger (32.9 vs. 6.7).
TABLE 4comparison of the oblique optical performance of mode 1 ofGuo's single-polarizer reflective 2πBTN display for thetwo different polarizer configurations.Config. No.P angleθmaxΔCmax#124.24°43°32.9#2−65.76°32°35.0
The optical performance of configuration 1 is presented in FIG. 13 where it is seen that the optical performance of Guo's 2πBTN mode 1 is inferior to the corresponding intrinsic πBTN case shown in FIG. 8. Furthermore, the contrast ratio for perpendicular viewing is only 33 for mode 1 of the 2πBTN display whereas it is 72 for mode 1-1 of the πBTN display.
Influence of TAC Film
Adding a TAC layer to the mode 1 2πBTN display further worsens its oblique viewing performance. FIG. 14 shows a 2πBTN optical stack comprising a reflector 32, a 2πBTN liquid crystal layer 38 with retardation Δn·d=310.8 nm and twist angles of −67.2° and +292.8°, a −55 nm TAC layer 25 and a polarizer 24 with P 24.24°. The viewer 20 is positioned on the opposite side of the reflector with respect to the liquid crystal layer 38.
Table 5 presents θmax and ΔCmax for the stack of FIG. 14. Configuration 1 with the 24.24° polarizer angle gives the best performance of the two.
TABLE 5comparison of the oblique optical performance of mode 1 ofGuo's single-polarizer reflective 2πBTN display withone TAC layer for the two different polarizer configurations.Config. No.P angleθmaxΔCmax#124.24°38°37.9#2−65.76°31°39.2
The optical performance of configuration 1 is presented in FIG. 15.
Optical Compensation of BTN Displays
Osterman (IDW '02, p 101-103) proposed placing a simple uniaxial quarter-wave retardation film (in plane (x,y) retardation equal λ/4, no out of plane (x,z) retardation because uniaxial) with optical axis making an angle of 45° with respect to the liquid crystal director situated on the viewer side of the device, either inside or outside a reflective πBTN display. This birefringent film is an exact quarter wave plate, which transforms linear polarization into circular one, because of retardation value λ/4 combined with angular positioning between optical axis and entrance polarizer of 45°. The best transformation is obtained when the retardation λ/4 is taken with green light (maximum eye sensitivity), in this case λ=550 nm. The object of Osterman's work was to optimize a single-polarizer reflective πBTN display for high contrast ratio and high brightness at normal incidence viewing. No attempt was made to optimize the retarder for high contrast ratio or reduced color shift at oblique viewing angles.
The principle of optical compensation of 2πBTN LC mode is suggested in patent EP 1170624 from Rolic. This patent is related to domain stabilized 2πBTN with specific patterned alignment layer. In §[0067], it is explained that the invention can be combined with “optical compensation layers” which can be “liquid crystalline thin films (uniaxial or twisted), stretched polymer films or of combinations of such films”, “to improve brightness and/or contrast”. Nothing more is described concerning the said compensation film, the sentence has a very general purpose and is obvious, because it is well known that any LC mode, including 2πBTN, can be optically improved by the use of compensation film. Moreover, this patent is related to a 2πBTN device comprising two polarizers, as it is said in the abstract.
As we previously said, single-polarizer reflective BTN displays with conventional birefringent retarders are known, but the retarders have not been optimized with the intent of improving the viewing angle. Kwok, for example, (Journal of Applied Optics vol. 88 No. 4 p 1718) published adding a quarter wave positive A plate to the reflector side of a 2πBTN display cell so that it would operate with high contrast. Similarly, Guo (Applied Optics vol. 42 No. 19, 2003, p 3853) proposed adding a plurality of half-wave and full-wave plates to the observer side of a 2πBTN display cell to obtain a high contrast, single-polarizer reflective display. The Guo birefringent film, which can be waveplate (retardation=λ) or half waveplate (retardation=λ/2), are, like for Ostermann case, defined with λ in the green area. The half wave has the function to rotate a linear polarization, and the best mode is when the half wave plate is defined with green light. So Guo uses birefringent film with retardation 550 nm and 275 nm (see p 3856).
The goal of these compensations was to improve the contrast ratio and reflectivity for observation at normal incidence. No attempt was made to optimize for high contrast ratio or reduced color shift at oblique viewing angles. Guo only shows that the viewing-angle properties of his compensation scheme designed for normal incidence remains almost unchanged compared to the case without compensation for one case, and clearly explain that the viewing angle performance is “slightly narrower for compensated mode” (see p 3860). So the skilled person has no incitation to consider this document to improve viewing angle performance at oblique incidence of 2πBTN displays.
U.S. Pat. No. 6,765,640 also calculates a retarder optimized for reflective 2πBTN display at normal incidence. Like in the other publications, half and quarter wave plates are considered, also defined with green light: the used half wave plate is equal to 270 nm (540 nm/2; column 7) and the quarter wave plate is equal to 132.5 nm (530/4; column 8). Optical performances are only given at normal incidence.